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Deciphering AIX Volume Group limitations and types

Modern versions of AIX have three types of Volume Groups: Original, Big, and Scalable.

In my opinion, for any new volume groups you are creating you should always use scalable unless you will have the need to export the volume group and import it on very old versions of AIX. Scalable volume groups have very few limitations when compared to big and original volume groups.

Unfortunately there are a ton of original and big volume groups out there in the world, and it can be very tricky to understand their limitations. The main barriers people hit with original and big volume groups is the limit on the maximum number of disks in the volume group ("MAX PVs") and the limit on how many PP's can be on the disk which directly impacts how large a disk in the volume group can be ("MAX PPs per PV").

To further complicate this, these 2 limitations are variable based on the volume group "Factor". The factor is a number between 1-16 for original volume groups, and between 1-64 for big volume groups. As you increase the factor, the "MAX PVs" does down and the "MAX PPs per PV" goes up. It is essentially a trade off: if you would like larger disks, you can't have as many. If you want more disks, they can't have as many PP's on them.

The factor of a volume group can be changed with the "chvg -t" command.

Below is how the factor affects original VG's and Big VG's (Scalable VG's don't have a factor setting):

Original VG

Factor

Max PV's

Max PP's per PV

1

32

1016

2

16

2032

3

10

3048

4

8

4064

5

6

5080

6

5

6096

7

4

7112

8

4

8128

9

3

9144

10

3

10160

11

2

11176

12

2

12192

13

2

13208

14

2

14224

15

2

15240

16

2

16256

Big VG

Factor

Max PV's

Max PP's per PV

1

128

1016

2

64

2032

3

42

3048

4

32

4064

5

25

5080

6

21

6096

7

18

7112

8

16

8128

9

14

9144

10

12

10160

11

11

11176

12

10

12192

13

9

13208

14

9

14224

15

8

15240

16

8

16256

17

7

17272

18

7

18288

19

6

19304

20

6

20320

21

6

21336

22

5

22352

23

5

23368

24

5

24384

25

5

25400

26

4

26416

27

4

27432

28

4

28448

29

4

29464

30

4

30480

31

4

31496

32

4

32512

33

3

33528

34

3

34544

35

3

35560

36

3

36576

37

3

37592

38

3

38608

39

3

39624

40

3

40640

41

3

41656

42

3

42672

43

2

43688

44

2

44704

45

2

45720

46

2

46736

47

2

47752

48

2

48768

49

2

49784

50

2

50800

51

2

51816

52

2

52832

53

2

53848

54

2

54864

55

2

55880

56

2

56896

57

2

57912

58

2

58928

59

2

59944

60

2

60960

61

2

61976

62

2

62992

63

2

64008

64

2

65024

The standard AIX utilities such as "lsvg" don't directly show you the volume group type or the volume group factor. There is a "readvgda" command that you can run on a hdisk (i.e. "readvgda hdisk0") that will show the volume group type and the factor, but it also shows a bunch of other info so its difficult to quickly and efficiently get information from.

I've written a script that produces a quick report showing several things such as the volume group type, factor size, max PV's, used PV's, Max PP's per PV, PP Size, and max disk size.